It has long been recognized that synchronizing atrial and ventricular cardioversion pulses to native depolarizations in the chamber being treated improves efficacy of treatment. For example, synchronization of ventricular cardioversion shocks to sensed R-waves is disclosed in U.S. Pat. No. 4,375,817 issued to Engle et al. Synchronization of cardioversion shocks intended to treat atrial or ventricular tachycardia or fibrillation to detected R-waves is disclosed in U.S. Pat. No. 4,384,585, issued to Zipes. Similarly, synchronization of atrial cardioversion shocks to detected P-waves is disclosed in U.S. Pat. No. 4,572,191, issued to Mirowski et al.
Delivery of cardioversion or defibrillation shocks intended to terminate a tachyarrhythmia in one chamber may unfortunately induce a tachyarrhythmia in the other chamber. The risk associated with tachyarrhythmia induction in the ventricle is sufficiently great that it has long been recognized that atrial defibrillation pulses need to be timed to avoid the vulnerable period of the ventricle. The most common approach to accomplish this result has been to deliver the atrial defibrillation or cardioversion pulse closely synchronized to a sensed ventricular depolarization to avoid the associated ventricular vulnerable period, as disclosed in U.S. Pat. No. 4,384,585, issued to Zipes. It has also long been recognized that the vulnerable period following a ventricular depolarization may extend to include the subsequent ventricular depolarization in the presence of a sufficiently rapid ventricular rhythm. In such cases, there is no safe time to deliver a cardioversion pulse, as discussed in the article “Synchronous Intracardiac Cardioversion”, by Zipes et al., published in Modern Cardiac Pacing, edited by Barold, Futura Publishing Co. 1985, pages 727–743.
Because cardioversion pulses synchronized to a ventricular rhythm that is too rapid may induce ventricular arrhythmias or fibrillation, implantable cardioverters have typically included some method to assure that a minimum R-R interval has elapsed as a prerequisite to delivery of a cardioversion shock. One such synchronization method which prevents delivery of a cardioversion pulse, synchronized to a ventricular rhythm which is too rapid, is to require that the shock be synchronized to a ventricular depolarization falling outside a defined refractory period that follows the preceding ventricular depolarization, as in the Model 7210 implantable transvenous cardioverter manufactured by Medtronic, Inc. While this device could sense ventricular depolarizations during this refractory period and would initiate a new refractory period following such depolarizations, it would not deliver cardioversion pulses synchronized to such depolarizations. As reflected in the above-cited article by Zipes et al, the transvenous cardioversion therapy provided by the model 7210 device could be employed to treat either ventricular or supraventricular tachyarrhythmias.
A more sophisticated method of synchronization to sensed R-waves is set forth in U.S. Pat. No. 5,486,198, in which a shock is delivered synchronized to an R-wave only if the interval between the R-wave and the immediately preceding R-wave is greater than or, no more than, a defined amount less than the immediately preceding R-R interval. Unfortunately, this method of synchronization to sensed R-waves, like those discussed above, does not allow for the safe delivery of an atrial cardioversion shock in the presence of too rapid a ventricular rate.
An alternative method for preventing the delivery of an atrial cardioversion shock during the ventricular vulnerable period is to deliver the shock after a defined interval that follows a preceding R-wave, in the absence of an intervening sensed ventricular depolarization-the defined interval being of sufficient duration to prevent delivery during the vulnerable period associated with the preceding R-wave. Such a synchronization method is disclosed in U.S. Pat. No. 5,411,524, issued to Mehra. As disclosed in the Mehra patent, the defined interval may vary as a function of the sensed ventricular rate, but it must be greater than a predefined minimum duration and thus is also unavailable in the presence of too rapid a ventricular rate.
An additional method to avoid delivery of a cardioversion pulse during the vulnerable period of a chamber of the heart is to pace the chamber and deliver the cardioversion pulse during the absolute refractory period following the pacing pulse. One such approach is also disclosed in U.S. Pat. No. 5,411,524 issued to Mehra, wherein an atrial cardioversion pulse is synchronized to a single ventricular pacing pulse and in U.S. Pat. No. 5,193,536, also issued to Mehra, wherein a ventricular cardioversion pulse is synchronized to the last of a series of ventricular pacing pulses. In both cases, the ventricular pacing escape interval is calculated to be less than the intervals separating intrinsic ventricular depolarizations. These methods unfortunately are not necessarily useful in the case of a ventricular rhythm that is so rapid as to render the synchronization to an overdrive ventricular pacing pulse unsafe.